1. Field of the Invention
This invention relates to liquid metal ion sources, and more particularly, to the wetting of boron bearing ion source alloys to portions of an ion source structure.
2. Description of Related Art
Liquid metal ion sources provide high current densities of ions from a source having a small virtual source size. A high brightness and small source size are required when the ion beam is to be focused with a high resolution of, for example, less than 1 micrometer spot size, and utilized in applications such as the fabrication of semiconductor microcircuits. The high current density and small virtual source size are achieved by emitting the ions from a source structure embodying a sharp point, such as the point of a needle. In one such approach, a needle is covered with a layer of liquid ion source metal, and a cusp in the liquid metal at the point of the needle is created by application of an electrostatic field. This fine cusp then becomes the emitting source for the extraction of ions. As the ions are emitted from the source, more liquid source metal must flow from the reservoir down the needle to the cusp to replenish that which has been extracted.
For this type of high brightness ion source to operate properly, the ion source alloy must wet the needle to ensure a smooth flow of metal from the reservoir to the cusp. If the ion source alloy does not wet the needle, ions cannot be extracted from the emitter tip. If the needle is incompletely wetted, flow of the liquid alloy may be insufficient to maintain a constant ion beam current, or flow may occur asymetrically resulting in the formation of an offset, illformed cusp. Such a cusp manifests itself as a large or non-circular source of ions. The presence of any of these problems results in poor performance of the source.
In some instances, ion source alloys naturally wet the needle. The wetting reaction may sometimes be so complete that the ion source alloy can rapidly corrode the wetted portions of the ion source structure, leading to a relatively short operating time before the ion source must be replaced. Such corrosion changes the geometry of the emitter tip as well as the composition of the alloy being ion evaporated, both effects serving to reduce the efficiency of the ion source. Obtaining a suitable material for making the wetted components of an ion source structure, typically, involves balancing the properties of sufficient wetting of the structure by the source alloy to attain metal flow during ion evaporation, and the inhibition of adverse corrosive effects. Briefly, raising the ion source temperature above the normal operating temperature of the source often improves the wetting, but this approach is of limited use because of the associated increase in corrosion rate at elevated temperatures as well as the increased rate of loss of volatile elements which may be present in the ion source alloy.
The ion evaporation of boron from boron-containing source alloys is particularly important in the field of doping by ion implantation in silicon semiconductors. Molten alloys containing the metalloid boron do not wet many candidate emitter materials thoroughly, and corrosively attack most that are wetted.
Boron is typically ion evaporated from boron-platinum, boron-palladium, boron-nickel, boron-containing ternary alloys or more complex alloys containing boron. Boron-platinum alloys have been used for sometime with ion source structures whose wetted components are composed of pure rhenium. An average lifetime of 10 hours is realized with such a combination. The source structure ultimately fails because of the gradual formation of the ternary alloy boron-platinum-rhenium, which requires a constantly increasing temperature in order to maintain liquidity. The entire structure finally becomes unstable so that either the tip of the emitter is pulled off by the electrostatic field or the heater-reservoir burns out.
In another approach, a glassy carbon emitter needle is used to ion evaporate boron from nickel-boron alloys, with the source having a usable lifetime of about 50-90 hours. It has been reported, however, that the nickel constituent of the ion source alloy ultimately corrodes the glassy carbon emitter. It is probable that the presence of nickel in these alloys facilitates wetting the alloy to the glassy carbon substrate.
Longer source lifetimes are desired, and longer lifetimes can be achieved by utilizing alloys having lower melting points and operating the source at the minimum temperature required for ion extraction. The eutectics of the alloys mentioned above have melting points between 800.degree. C. and 900.degree. C., but at such temperatures metal source structures are rapidly attacked by the boron or nickel constituent. There exists the possibility of lower melting point alloys containing boron, but noen has yet been reported. There also exists the possibility of using alloys containing an ingredient which inhibits corrosion, but again none has yet been reported.
Consequently, there exists a need for an improved ion source for ion evaporation of boron from boron-containing alloys with relatively high melting points in the range of at least 800.degree. C. to 900.degree. C. The approach could involve modification of the source alloy, selection of a different material for fabricating the source structure, modification of the source structure, a combination of these approaches, or some entirely new technique. Whatever the technique chosen, it should allow the wetting of the source emitter and other components of the ion source contacted by the source metal, by a boron-containing source alloy without extensive corrosion of the source structure. No reaction between the source alloy and source structure should occur to modify the metallurgy of the source alloy. Finally, the technique should not require the operation of the source at greatly elevated temperatures for long periods of time when contacted with the alloy so that volatile componets, which may be present in the alloy, are evaporated. The present invention fulfills this need, and further provides related advantages.